The present invention relates to nozzle segments for use in gas turbines employing a closed circuit cooling system and particularly relates to an exit chimney joint with the outer band of a nozzle segment and methods of forming the joint.
In current gas turbine designs, nozzle segments are typically arranged in an annular array about the rotary axis of the turbine. The array of segments forms outer and inner annular bands and a plurality of circumferentially spaced, generally radially extending vanes extend between the bands. The bands and vanes define in part the hot gas path through the gas turbine. Each nozzle segment comprises an outer band portion and an inner band portion and one or more of the nozzle vanes extend between the outer and inner band portions. In current gas turbine designs, a cooling medium, for example, steam, is supplied to each of the nozzle segments to cool the parts exposed to the hot gas path. To accommodate the steam cooling, each band portion includes a nozzle wall in part defining the hot gas path through the turbine, a cover radially spaced from the nozzle wall defining a chamber therewith and an impingement plate disposed in the chamber. The impingement plate defines with the cover a first cavity on one side thereof for receiving cooling steam from a cooling steam inlet. The impingement plate also defines, along an opposite side thereof and with the nozzle wall, a second cavity. The impingement plate has a plurality of apertures for flowing the cooling steam from the first cavity into the second cavity for impingement cooling the nozzle wall. The cooling steam then flows radially inwardly through cavities in the vane(s), certain of which include inserts with apertures for impingement cooling the side walls of the vane. The cooling steam then enters a chamber in the inner band portion and reverses its flow direction for flow radially outwardly through an impingement plate for impingement cooling the nozzle wall of the inner band portion. The spent cooling medium flows back through cavities in the vane to an exhaust port of the nozzle segment.
It will be appreciated that in a current steam cooled nozzle segment design of the assignee hereof, an exit chimney is provided for flowing the spent cooling steam from the vane cavities past the nozzle wall, impingement plate and cover of the outer band portion to an exit port coupled to the outer band. In a current design, an exit chimney is integrally cast with the cover and extends radially inwardly into the exit openings of the cavities flowing the spent cooling steam radially outward. The integral exit chimney is first brazed about its margin to a margin of the impingement plate. Subsequently, this sub-assembly is brazed with the margin about the exit openings of the vane cavities. The primary joint between the nozzle wall about the vane cavities and the exit chimney requires a certain robustness due to the stresses across the joint and to the pressure difference on opposite sides of the exit chimney walls. It will be appreciated that the exit chimney on one side is exposed to inlet high pressure cooling steam and, on its opposite side, to exit hotter lower pressure spent cooling steam. There are also thermal and mechanical stresses that are taken up through the exit chimney.
In this current design, however, the primary joint between the exit chimney and nozzle wall is through a pair of brazed joints, one of which necessarily has to be formed as a blind joint. That is, the joint between the impingement plate and the nozzle side wall which is critical to the system must be formed after the sub-assembly of the cover and impingement plate is applied to the side wall. Thus, the current method of forming the joint results in a joint which is not robust and may be of reduced quality because it must be accomplished blindly.
Additionally, the joints of the current design cannot be inspected after fabrication. This is particularly important because brazed joints require finite gaps for proper brazing. If the manufacturing tolerances vary, e.g., 10-20 mils, a required gap appears only problematically because the final brazing gap is unknown and too large. Consequently, there is a need to improve the joint between the nozzle casting, the exit chimney and the nozzle cover in order to enhance the stress carrying capability of the joint and its capacity to be inspected and produced with consistency.
According to a preferred embodiment of the present invention, the exit chimney is provided as a discrete part serving as a flow channel for receiving the spent cooling medium from the vane cavities and transmitting the cooling medium past the nozzle wall, impingement plate and cover into the exit port. The configuration of the exit chimney which may be a casting or fabrication and corresponding configuration of the nozzle wall, impingement plate and cover provide for both accessible and inspectable joints prior to and after the joints are formed. To accomplish the foregoing, the exit chimney is in the form of an endless sleeve shaped for reception in the margins of the vane cavity walls surrounding the exit cavities of the vane at one end with a generally corresponding configuration at its opposite end for forming end joints with the vane cavity wall and cover, respectively. A radially outwardly projecting rib is provided intermediate its ends for forming a joint with the impingement plate. Each of the three joints is accessible while being formed, is available for inspection after the joint is formed prior to forming the next joint and may be formed by brazing, E-beam or laser welding.
In accordance with this invention, the first joint is formed between the radial inner end of the exit chimney and a margin or rib surrounding the vane cavities which will deliver the spent cooling steam to the chimney for flow to the exit port. Thus, the inner end of the exit chimney is welded to the margin about the vane cavities from a location radially outwardly of the nozzle wall. This first joint can be formed as robust as necessary and is clearly accessible during and after formation. Subsequent to forming this first joint, the impingement plate is welded or brazed to a rib on the exit chimney intermediate opposite ends of the chimney. This joint likewise is accessible radially outwardly of the joint, both while forming the joint and subsequent to joint formation for inspection. Subsequent to forming the second joint, the third joint between the cover and the outer end of the exit chimney is formed. This joint likewise is accessible outwardly of the joint and can be inspected subsequent to joint formation. With the exit chimney joined to the nozzle segment and the cover applied, the exit port can be welded to the cover about the outer opening of the chimney, affording a through passage from the cavities, through the chimney, directly to the exit port.
In a preferred embodiment according to the present invention, there is provided in a gas turbine, a nozzle segment having outer and inner band portions and at least one vane extending between the band portions, one vane including at least first and second vane cavities, an exit chimney in the outer band portion in communication with the second vane cavity, at least the outer band portion including a nozzle wall defining in part a hot gas path through the turbine, a cover radially spaced from the nozzle wall of the outer band portion defining a chamber therebetween and an impingement plate disposed in the chamber to define with the cover a first plenum for receiving a cooling medium and with the nozzle wall a second plenum on a side of the impingement plate opposite the first plenum, the impingement plate having a plurality of apertures therethrough for flowing the cooling medium from the first plenum into the second plenum for impingement cooling the nozzle wall, the first vane cavity lying in communication with the second plenum for flowing the cooling medium along the vane to the inner band portion, the second vane cavity lying in communication with the inner band for flowing the cooling medium along the vane to the exit chimney, a first joint between one end of the exit chimney and margins of the vane about the second cavity, a second joint between the impingement plate and the exit chimney along the chimney intermediate one end and an opposite end of the chimney, the cover having an opening and a third joint at the opposite end of the exit chimney and the cover about the opening for flowing cooling medium through the exit chimney past the cover to an exit port.
In a further preferred embodiment according to the present invention, there is provided in a gas turbine having a nozzle segment comprised of outer and inner band portions and at least one vane extending between the band portions with at least one vane cavity extending along the vane, the outer band portion including a nozzle wall, a cover radially spaced from the nozzle wall defining a chamber therewith and an impingement plate in the chamber to define with the cover a first plenum for receiving a cooling medium and with the nozzle wall, a second plenum to receive cooling medium flowing through apertures in the impingement plate for impingement cooling the nozzle wall, a method of securing an exit chimney in the nozzle segment, comprising the steps of (a) securing one end of the exit chimney to the nozzle wall about a margin of the vane cavity at a first joint therebetween, (b) subsequent to step (a), securing margins of the impingement plate and the exit chimney to one another at a second joint along the chimney intermediate opposite ends of the chimney and (c) subsequent to step (b) securing an opposite end of the chimney and the cover to one another with the chimney in communication with the exit opening of the vane cavity.